Against the dressed size
—
Shop estimate. Measure your stock and cut a test on scrap.
Dressed softwood
| Nominal | Dressed inches | Inch size in mm | CLS mm |
|---|---|---|---|
| 1×2 | 0.75 × 1.5 | 19.0 × 38.1 | 19 × 38 |
| 1×3 | 0.75 × 2.5 | 19.0 × 63.5 | 19 × 64 |
| 1×4 | 0.75 × 3.5 | 19.0 × 88.9 | 19 × 89 |
| 1×6 | 0.75 × 5.5 | 19.0 × 139.7 | 19 × 140 |
| 1×8 | 0.75 × 7.25 | 19.0 × 184.1 | 19 × 184 |
| 1×10 | 0.75 × 9.25 | 19.0 × 234.9 | 19 × 235 |
| 1×12 | 0.75 × 11.25 | 19.0 × 285.8 | 19 × 286 |
| 2×2 | 1.5 × 1.5 | 38.1 × 38.1 | 38 × 38 |
| 2×3 | 1.5 × 2.5 | 38.1 × 63.5 | 38 × 64 |
| 2×4 | 1.5 × 3.5 | 38.1 × 88.9 | 38 × 89 |
| 2×6 | 1.5 × 5.5 | 38.1 × 139.7 | 38 × 140 |
| 2×8 | 1.5 × 7.25 | 38.1 × 184.1 | 38 × 184 |
| 2×10 | 1.5 × 9.25 | 38.1 × 234.9 | 38 × 235 |
| 2×12 | 1.5 × 11.25 | 38.1 × 285.8 | 38 × 286 |
| 4×4 | 3.5 × 3.5 | 88.9 × 88.9 | 89 × 89 |
| 4×6 | 3.5 × 5.5 | 88.9 × 139.7 | 89 × 140 |
| 6×6 | 5.5 × 5.5 | 139.7 × 139.7 | 140 × 140 |
Hardwood quarters, rough and a common S2S target
| Quarter | Rough | Typical S2S |
|---|---|---|
| 4/4 | 1 in (25.4 mm) | 13/16 in (20.6 mm) |
| 5/4 | 1.25 in (31.8 mm) | 1 1/16 in (27.0 mm) |
| 6/4 | 1.5 in (38.1 mm) | 1 5/16 in (33.3 mm) |
| 8/4 | 2 in (50.8 mm) | 1 3/4 in (44.4 mm) |
| 12/4 | 3 in (76.2 mm) | 2 3/4 in (69.8 mm) |
CLS millimetre sizes are standardised sizes, not the inch fraction times 25.4. Hardwood surfacing is a dealer habit. Measure the board.
How to use it
- Pick the nominal size printed on the stick or the rack tag.
- Read the dressed inch size and the CLS millimetre size. They are not always the same number of millimetres, because the metric sizes are standardised on their own.
- If you have calipers on the board, type the thickness and width you measured. The difference is measured size minus the dressed standard.
- For hardwood, use the quarter table. 4/4 means a rough thickness of one inch. A surfaced face is whatever the dealer took off. Measure it.
The formula
Difference = measured size − dressed size. Positive means the board is over the standard.
- PS 20. The American Softwood Lumber Standard. Dressed dry 2×4 stock is 1½ by 3½ inches.
- CLS. Canadian dimension lumber dressed to standard millimetre sizes, such as 38 by 89 for a 2×4.
- 4/4. A hardwood count of quarter inches of rough thickness. 4/4 is 1 inch rough, not a promise of 1 inch after surfacing.
Worked example
Select 2×4. The dressed dry size under the American softwood standard is 1½ by 3½ inches. The Canadian CLS millimetre size is 38 by 89, which is the standard mill size, not a conversion of 3.5 inches (that conversion is 88.9 mm).
Type a measured width of 3.4 inches and the comparison shows the board is 0.1 inch under the dressed width. That is a check on the stick, not a reason to send it back without looking at the grade stamp and the moisture.
What it will not decide
- Green lumber can be dressed to a different size than dry lumber. The table is the dry dressed size.
- Hardwood widths are whatever the board was sawn. Surfacing changes thickness much more than it standardises width.
- Nothing here is a grade. Wane, skip, and moisture are not in the arithmetic.
Questions
Why is CLS 89 mm and not 88.9 mm?
3½ inches times 25.4 is 88.9 mm. Canadian dressed sizes are defined in whole millimetres. A 2×4 is 38 by 89 mm. Use the column that matches how the lumber was sold, and measure if the joint cares about a tenth of a millimetre.
What thickness should I expect from 4/4 hardwood?
Rough, about 1 inch. Surfaced two sides, dealers often aim near 13/16 inch. Some will dress to 3/4. The table calls 13/16 a common target, not a rule. The board in the rack is the measurement that counts.
Does this cover stair stringers or joist spans?
No. Span, notch, and stair sizes belong to the building code and to someone stamping drawings. This page is the size of the stick after the planer, nothing structural.